The worldwide citrus industry is currently battling with two potentially devastating diseases: citrus greening, also known as Huanglongbing or HLB, and citrus canker.
Citrus greening is the most destructive, highly infectious disease of most commercial citrus varieties. This disease is caused by the Gram-negative HLB bacterium that belongs to the genus Candidatus Liberibacter as reported by J. M. Bove in “Huanglongbing: A destructive, newly-emerging, century-old disease of citrus,” Journal of Plant Pathology 2006, 88, (1), 7-37. HLB threatens the citrus industry worldwide and may cause damage to the citrus industry and economy in the State of Florida. The problem can be severe and kill citrus trees. The disease causes fruits to taste bitter and become deformed, small-sized and poorly-colored, making it unusable and unmarketable. Currently, there is no cure for the HLB.
The Asian Citrus Psyllid (ACP) is an invasive phloem-feeding insect that causes serious damage to citrus plants and citrus plant relatives. Burned tips and twisted leaves result from an infestation of ACP on new growth. In addition, ACP is a vector of HLB and carries HLB and can rapidly spread the disease from one grove to another.
Integrated pest management strategies that include the use of disease-free nursery trees, rapid removal of symptomatic trees and aggressive control of ACPs using foliar insecticides have been the primary focus to manage HLB infected groves according to A. Morris et al. in “Economic tradeoffs of citrus greening management” Citrus Industry 2008, 89 (4), 26-28 and P. L. Hollis in “Scientists combining efforts to combat greening in Florida's citrus industry” Southeast Farm Press 2008, 35 (15), 35.
Unfortunately, the use of foliar insecticides appears to be the only solution available to growers these days to prevent HLB infection, even though such integrated practices are expensive and labor extensive.
Citrus canker is another serious disease, caused by the bacterium Xanthomonas axonopodis pv. citri, that affects most commercial citrus varieties and has caused economic loss world-wide according to T. R. Gottwald in “The citrus canker epidemic in Florida: The scientific basis of regulatory eradication policy for an invasive species,” Phytopathology 2001, 91, (1), 30-34. Canker causes necrotic lesions on various parts of trees that include fruits, leaves and stems. The severity of the disease or infection is manifested as defoliation, premature fruit drop, blemished fruit and general tree decline as reported by J. H. Graham in “Xanthomonas axonopodis pv. citri: factors affecting successful eradication of citrus canker,” Molecular Plant Pathology 2004, 5, (1), 1-15 and A. K. Das in “Citrus Canker—A review,” J. Appl. Hort. 2003, 5 (1), 52-60.
While there is no cure for the HLB disease, canker losses have been controlled by the use of appropriate anti-bacterial agents such as copper (Cu) based compounds, including, but not limited to, Cu oxychloride, Cu sulphate, Cu hydroxide, Cu oxide, ammonia-Cu carbonate, antibiotics, such as, streptomycin, tetracycline, and induced systemic resistance compounds, including, acibenzolar-S-methyl, harpin protein. To date, copper (Cu) has been the gold standard for controlling citrus canker disease worldwide due to its effectiveness in protecting against the possibility of infection and minimal development of Cu resistance by a pathogen.
Due to destructive nature of HLB and citrus canker diseases, there is a need to find solutions to combat HLB and citrus canker diseases.